This invention relates to a kind of biodegradable product, and in particular to a biodegradable polystyrene foaming product, particularly to biodegradable polystyrene foam sheet and polystyrene containers made therefrom such as in fast food containers.
Polystyrene sheet and their products (such as lunch boxes, bowls, dishes, etc.) presently supplied in the domestic market are all non-degradable products, and are often discarded to dumps after utilization, only a small part of such products are recovered, regenerated, and treated. Most of the products remain and thus may form a public hazard of "plastic wastes", cause environmental pollution and influence ecological equilibrium.
Biodegradable plastic foaming products are known, the raw materials used involving a starch and some synthetic polymers selected from ethylene vinyl alcohol, ethylene-acrylic acid and their mixtures. These products possess an obdurate structure with a density of 0.1-0.3 g/cm.sup.3. Production of such biodegradable foaming products involves an extrusive component of a starch and synthetic polymers selected from ethylene vinyl alcohol and ethylene acrylic acid, and carried out in the presence a foaming agent formed from reaction of a polymeric acid and sodium bicarbonate.
U.S. Pat. No. 5,880,184 describes the extrusion of a starch, such as sago starch, with a water-soluable polymer. In U.S. Pat. No. 5,496,895, a biodegradable plastic material, that can include polystyrene, polyurethane, polyethylene, polypropylene, or polycarbonate, can be prepared with a carbohydrate, such as a starch, and adding a compound to degrade the carbohydrate. Glycerol is among more than fifty diverse carbohydrate-degrading compounds listed.
Since starch-based plastics are accepted by the public, the species of products and the application fields have been increased and expanded gradually, but the Polystyrene sheet and products prepared by starch used as a filler can only be produced by a few producers. Moreover, because of the limitation of production technology and equipment, the mixed amount of starch is less than 10%. At such levels the starch does not have a significant degradable effect. It can only be regarded as a means of lowering the cost of raw material and cannot really attain the aim of popularizing the application of degradable polystyrene material. Besides, in the course of sheet drawing, on account of the incorporation of starch, there are still many shortcomings in the mastering of formulations and technology. Difficulties experienced with grafting of starch with polymer and the appearance of semi-products and products so produced, have prevented the widespread use of starch.